Pressure swing oxygen concentrators are frequently used to produce oxygen for medical purposes. A typical pressure swing oxygen concentrator has at least 2 molecular sieve beds which alternately operate to separate nitrogen from air, producing an oxygen enriched gas suitable for medical use. A compressor is connected through a valve to supply a flow of pressurized air to the operating molecular sieve bed which functions as a filter by passing a flow of oxygen and blocking the flow of nitrogen. With time, the operating sieve bed becomes clogged with the separated nitrogen. As the sieve bed becomes clogged, the pressure drop across the sieve bed increases. The increased pressure is necessary to maintain the maximum flow of oxygen enriched gas to maintain the maximum efficiency in gas production. If a sieve bed is operated too long, it will become saturated with and will pass nitrogen, reducing the oxygen concentration of the product gas.
Once the operating sieve bed becomes saturated with nitrogen, valves are operated to connect a different sieve bed to the compressor for producing a flow of oxygen enriched gas and the saturated sieve bed is switched to a purge mode. In the purge mode, the inlet to the sieve bed is vented to atmosphere. The outlet sides of the sieve beds are connected together through a flow restricting orifice which allows a limited flow of pressurized oxygen enriched product gas to flow to the outlet end of the sieve bed in the purge mode to flush nitrogen from the saturated sieve bed. After nitrogen is purged from the sieve bed, the vented inlet side may be closed to allow the pressure to equalize between the sieve beds before the purged bed is switched to the separation mode.
When an oxygen concentrator is operated to produce a maximum flow of oxygen enriched gas, it is not energy efficient. The sieve beds are switched between the separation mode and the purge mode only when necessary. As the separation cycle progresses in a sieve bed, the pressure drop across the sieve bed increases, simultaneously increasing the load on the compressor and the energy required to drive the compressor. However, most patients who require medical oxygen do not require the maximum output flow from the concentrator. For example, an oxygen concentrator may have a 5 liters per minute flow rate, and the patient may only need 2 or 3 liters per minute of supplemental oxygen. An oxygen concentrator operating on a fixed cycle time will require the same energy input for the lower patient flow requirement as for the maximum flow rate, since the compressor produces the same maximum pressure regardless of the oxygen enriched gas flow needed by the patient. The compressor is the most significant energy user in an oxygen concentrator.
The prior art has suggested using a variable speed motor in the compressor in order to reduce the energy required to operated the compressor. However, this requires either a D.C. motor or a variable frequency control, both of which are costly. By slowing down the compressor without changing the cycle time, the sieve bed will have a lower maximum pressure in each cycle. However, if the compressor speed is reduced too much, the oxygen concentration in the product gas also will be reduced.
U.S. Pat. No. 4,272,265 teaches the use of a rotary valve driven by a constant speed motor for controlling the molecular sieve bed cycle in an oxygen concentrator. However, this patent is not concerned with the energy efficiency of the concentrator compressor.